Turbine blades for gas turbine engines operate in relatively harsh environments in which high velocity and high temperature gases erode the blade surfaces. Exotic alloys are used to form such blades in order to withstand the temperature extremes. Additionally, the blades are coated with other alloys or metals to improve surface resistance to ablation. It is desirable that the surface coatings on such blades be uniform and have a preselected thickness. It is therefore desirable to have some nondestructive method of confirming uniformity and thickness of coatings on such blades.
One nondestructive test procedure for measuring thickness of coatings on a substrate utilizes X-ray spectrometry and is described in ASTM Standard B 568-85. In X-ray spectrometry, the measurement of coating thickness is based upon the combined interaction of the coating and substrate with incident radiation of sufficient energy to cause the emission of secondary radiations characteristic of the coating and substrate. As is well known, when an element of a particular atomic number is excited by radiation, the secondary emission of radiation from that element will be characteristic of that element. Spectral analysis of the secondary radiation can be used to identify the emitting element. Furthermore, the amount of secondary radiation within certain limits is proportional to the thickness of the element when the exciting radiation is of constant intensity.
ASTM Standard B 568 outlines procedures for determining coating thickness using the basic premises discussed above. The procedures include one based upon secondary radiation absorption and one based upon secondary radiation emission. In the procedure using secondary X-ray emission detection, a spectrometer is positioned to record the intensity of a prominent wavelength characteristic of a coating metal or, in the case of an energy dispersive system, a multi-channel analyzer may be set to accept the range of energies comprising the desired characteristic emission. The intensity of the coating's X-ray emission will be at a minimum for a sample of the bare substrate. The detected emissions may consist of that portion of the substrate fluorescence which may overlap the fluorescence of the coating and any contributions due to background radiation. If the characteristic emission energies of the coating and substrate are sufficiently different, the only contribution of the substrate to the detected emission will be due to background. For a thick sample of solid coating metal or for a plated specimen having an infinitely thick coating, the secondary radiation intensity emitted will have its maximum value. For a sample having a coating of less than "infinite" thickness (actually a finite value less than a predetermined maximum), the intensity will have some intermediate value. The intensity of the emitted secondary X-ray radiation depends, in general, upon the excitation energy, the atomic numbers of the coating and substrate, the area of the specimen exposed to the primary radiation, the power of the X-ray tube, and the thickness of the coating. If all of the other variables are fixed, the intensity of the characteristic secondary radiation can be made a function of the thickness of mass per unit area of the coating. The exact relationship between the measured intensity and the coating thickness must be established by the use of standards having the same coating and substrate compositions as the samples to be measured. In a typical application, measurements are taken of a sample in such a manner as to establish a minimum intensity of the coating material emission for a bare substrate and a maximum intensity for a coating of infinite thickness. The infinite thickness may be obtained by exposing a block of material formed of the coating material to the X-ray radiation and measuring the intensity of the secondary radiation from the block material. The minimum and maximum secondary emissions establish the end points of a curve in which the midpoints can now be found by exposing substrates having different thicknesses of coatings to the X-ray radiation and measuring the emitted secondary radiation. The samples of coated products may be destructively tested to determine the actual thickness of the coating. FIG. 1 illustrates a typical curve or characteristic of a metallic coating on a metallic substrate. Any intermediate values of coating thickness will fall somewhere on the curve so that the count rate of secondary radiation emission can be converted directly to a thickness of coating.
A second procedure for measuring coating thickness uses the technique of X-ray absorption. In X-ray absorption, the secondary emission detector is set to record the intensity of a selected emission characteristic of the metal of the substrate. The intensity of the secondary emissions from the substrate metal will be a maximum for a sample in which the substrate metal is not coated and the intensity will decrease with increasing coating thickness. In this instance, an infinite coating thickness will result in a minimum detectable secondary emission radiation. The reason for the decreasing detectable secondary emission is that both the exciting and secondary characteristic radiations undergo attenuation in passing through the coating. The measurement of a coating thickness using the X-ray absorption technique is not applicable if any intermediate coating is present because there are indeterminate absorption effects of the intermediate layer. FIG. 2 illustrates a typical characteristic relationship between coating thickness and intensity of a characteristic emission from a substrate using the X-ray absorption technique.
The use of X-ray spectrometry to measure coating thickness on aircraft gas turbine blades formed of high temperature alloys such as Rene metal (such as type N5 or R80) has not resulted in consistently accurate results. Commercially available X-ray spectrometry thickness gauges utilize information obtained from samples placed in the gauges to construct characteristic secondary emission curves for use in thickness measurements. These gauges are typically microcomputer devices which collate the data from samples and use the collated data to directly convert count rates on actual products to a direct readout of thickness of a coating. In using these commercially available thickness gauges, Applicant has found that the readings provided by these gauges have not corresponded to actual measurements taken of coating thicknesses using destructive testing.